Duplex printing forms an image on both sides of an image receiving substrate or sheet of paper. Duplex printing may be performed using a system that forms images on both sides of a sheet at a single transfer station. In some duplex printing systems, the sheet is inverted after the sheet has received a first image on a first side by passing the sheet through the transfer station. A second image is then formed on a second side of the sheet by passing the inverted sheet through the same transfer station.
It can be difficult to position the first image on the first side of the sheet in a manner that coincides with the position of the second image on the second side of the sheet. Registration of the first image with the second image is not always accurate because one or more registration errors offset the first image relative to the second image. For example, a page number printed on the bottom-center position of the first side of a two-sided, printed document should align exactly with the page number printed on the reverse side. However, in many instances, the page numbers are printed offset from each other. The offset of the page number on the second side of the sheet with respect to the page number on the first side of the sheet is the result of a registration error that is extremely undesirable, and considered unacceptable in various printing industries.
Although registering the two images on the front and backsides of the sheet of paper can be difficult, the alignment is essential in industries such as the offset printing industry. In this industry, duplex sheets are sometimes produced having a number of pages that compose a single, multi-page document. To create the multi-page document, the sheet of paper is printed with multiple images on the front and backside of a single composite sheet, and the images are aligned on the front and back of the single sheet of paper. The single composite sheet is subsequently folded and segmented into individual pages. Each of the images on a first side a sheet must therefore be registered with a corresponding image on a second side of the sheet before the sheet may be segmented into individual pages.
Specifically, the first image that is formed on the first side of the sheet and the second image that is formed on the second side of the sheet are positioned so that identical images printed on both sides of the sheet are coincident with each other. For example, two identical images printed on both sides of a sheet of paper may form mirror images of each other if each image is printed with no intentional offset from the other. Thus, an image on the front side of the sheet would appear to be in perfect or transparent registration with the corresponding image on the backside of the sheet.
To ensure proper registration, it is essential that the position of the image receiving substrate be precisely controlled. Active registration systems are well know that sense a position of a sheet, and operate to correct the position of the sheet, if necessary, before an image is transferred to the sheet. However, even if the position of the sheet is controlled, errors in magnification make achieving such transparent registration difficult. The errors can be attributed to the operating speed of an image carrier, such as a photoreceptor belt or drum. Magnification errors can also be attributed to the frequency at which a write clock or a pixel clock operates. Another source of magnification errors is the expansion or contraction of paper, coupled with variation in properties from sheet to sheet. In order to correct such magnification errors, the speed of the photoreceptor belt or drum, or other such device, is adjusted, and the pixel clock frequency is adjusted.